A potential customer has asked me to do some work on his little upright.

Upon inspection it seems that many of the hammer felts have come completely away leaving the bare wood core exposed.

Was thinking of re-gluing/clamping them back on but not so sure if it would be strong enough as the felt takes quite a bit of squeezing to hold it back it place on the hammer core.

Not 100% sure the best way to proceed here , the piano is not particularly good quality and i feel a complete rehammer would not be cost effective.

Any thoughts?

The options are probably a partial replacement of the hammers, or a complete replacement of the piano. The latter is probably the most cost-effective around here, but I do not know what the used piano market is like in your part of the world.

Thanks for your reply
Any idea what the cost would be to replace a full set ? i haven't embarked on changing a full set since my college days.

Supply houses, Schaff for instance, used to sell partial sets. If you order a partial set that will cover the range in question and replace only the affected hammers you'll have a supply of spares.

I used to do that before I ran across a few "parts" pianos and built up a good supply of spare hammers.